1. Field of the Invention
This invention relates to a control system for a single-phase pulse-width modulation (hereinafter stated as PWM) converter, and more particularly to a control system for a single-phase PWM converter which converts AC single-phase voltage to DC voltage under PWM control.
2. Description of the Related Art
In a conventional single-phase PWM converter control system, the modulation frequency of pulse-width modulation was constant. As shown in FIG. 28, this was because a triangular wave generation unit 2 for pulse-width modulation was composed of a constant frequency triangular wave generation unit 29.
In order to improve efficiency of the PWM converter and design miniaturization of radiators by reducing the PWM converter switching loss while keeping the modulation frequency of pulse-width modulation constant, it is effective to set the PWM converter modulation frequency to a low value. However, if the switching frequency is set low, the ripple of the converter AC side current will become larger and exceed the peak turn-off current of the PWM converter switching devices (such as GTOs: gate turn-off thyristors). Therefore, the addition of a current smoothing reactor or the like will be required, thus making the system bulkier.
As shown in FIG. 29, a prior art single-phase PWM converter control system 101 was composed of a DC link voltage control unit 111, a converter voltage reference operation unit 112 and a triangular wave comparison unit 122. A DC link voltage reference value VdcRef and a DC link voltage actual value Vdc were inputted to DC link voltage control unit 111. It obtained a converter AC side current amplitude .vertline.Is.vertline. from their deviation and applied that to converter voltage reference operation unit 112. Converter voltage reference operation unit 112 obtained a converter power source voltage reference value Vc by inputting converter AC side current amplitude .vertline.Is.vertline., an AC voltage actual value Vs and an AC current actual value Is. It was designed then to perform pulse-width modulation by inputting converter power source voltage reference value Vc and a power source voltage sine wave phase .theta.s to triangular wave comparison unit 122.
That is to say, the pulse-width modulation of this single-phase PWM converter was performed by the triangular wave comparison between a high-frequency (about 500 Hz) triangular wave, 7-9 times higher than the AC power source voltage frequency (50-60 Hz).and the converter voltage reference value.
However, in such a prior art single-phase PWM converter control system, the current harmonics due to PWM converter switching resulted in electromagnetic noise in the AC power source transformer. In particular, in the case of a prior art PWM converter in which the switching frequency was about 500 Hz, 1 kHz-4 kHz noise, which is the most unpleasant sense of hearing, became greater.
Also, the lower the PWM converter/inverter DC link voltage was, the smaller the electromagnetic noise generated from the AC power source transformer and the motor which was the inverter load became. However, in the prior art triangular wave comparison PWM method, due to the limitation of the minimum ON time of a power switching device such as a GTO, the DC link voltage could not be set below a specified value determined from amplitude of the AC power source voltage.